(1) Field of the Invention
This invention relates to a microwave plasma torch apparatus which enables the production of a miniature plasma. The present invention also relates to a method for the use of the apparatus.
(2) Description of Related Art
Microwave plasma discharge source design and applications have been well developed over the past three decades. Various microwave plasma sources have been demonstrated with many gases over an operating range of pressures from 0.1 mTorr to several atmospheres, input powers from one watt to six kilowatts, and discharge diameters from 0.2 mm to 25 cm. Many basic microwave coupling and discharge structures have been investigated including quartz dome confined disk shaped plasmas at the end of resonant cavities, atmospheric microwave discharge jets/torch, stripline applicators for miniature microwave discharge generation and tubes through waveguides.
Some related art include: J. Asmussen, “Electron cyclotron resonance microwave discharges for etching and thin-film deposition,” J. Vac. Sci. Technol. A, vol. 7, pp 883-893 (1989). J. Asmussen, J. Hopwood, and F. C. Sze, “A 915 MHz/2.45 GHz ECR plasma source for large area ion beam and plasma processing,” Rev. Sci. Inst., vol. 6 pp 250-252 (1990). J. Asmussen, T. Grotjohn, P. Mak and M. Perrin, “The design and application of electron cyclotron resonance discharges,” Invited Paper for the 25th anniversary edition of the IEEE Trans. on Plasma Science, 25, 1196-1221 (1997). T. A. Grotjohn, A. Wijaya and J. Asmussen, Microwave Microstripline Circuits for the Creation and Maintenance of Mini and Micro Microwave Discharges, U.S. Pat. No. 6,759,808 issued Jul. 6, 2004. T. A. Grotjohn, J. Asmussen and J. Narendra, “Microstripline applicator for generating microwave plasma discharges,” 2003 NSF Design, Manufacturing and Industrial Innovation Conference Proceedings, Birmingham, Ala. 2003. J. Hopwood, D. K. Reinhard and J. Asmussen, “Experimental conditions for uniform anisotropic etching of silicon with a microwave electron cyclotron resonance plasma system,” J. Vac. Sci. Technol. B., vol. 6 1896-1899 (1988). J. Hopwood, D. K. Reinhard and J. Asmussen, “Charged particle densities and energy distributions in a multipolar electron cyclotron resonant plasma etching source,” J. Vac. Sci. Technol. A, vol. 8, pp 3103-3112 (1990). G. King, F. C. Sze, P. Mak, T. A. Grotjohn and J. Asmussen “Ion and neutral energies in a multipolar electron cyclotron resonance plasma source,” J. Vac. Sci. Technol. A, vol. 10, pp 1265-1269 (1992). P. Mak, G. King, T. A. Grotjohn, and J. Asmussen, “Investigation of the influence of electromagnetic excitation on election cyclotron resonance discharge properties,” J. Vac. Sci. Technol. A, vol 10, pp 1281-1287 (1992). S. Whitehair, J. Asmussen and S. Nakanishi, “Microwave electrothermal thruster performance in helium gas,” J. of Propulsion and Power, vol 3, pp. 136-144 (1987).
In a microwave torch, gas flows through a nozzle structure and microwave energy is introduced to create the discharge. Atmospheric pressure or near atmospheric pressure microwave plasma torches have application in the general areas of cutting, welding, toxic materials destruction, plasma-assisted CVD, plasma-assisted etching, surface treatment and materials heating. A variety of materials can be processed including metals, fiberglass, ceramics, and textiles.
In general, plasma torches can produce higher gas temperatures and/or higher reactive species than simple combustion processes. Microwave powered plasma torches have an advantage over transferred arc plasma sources in that the material being cut or processed does not need to be a metal. Hence, they can perform plasma cutting or processing on non-conducting materials such as ceramics and fiberglass, or on multi-layer materials. Also the ability to control the power level and flow rate yields a wide range of processing conditions. Applications range from gentle surface treatment for use in surface sterilization to intense torches for cutting very high temperature materials. Microwave sources/torches also have the advantage compared to DC electrode based systems that they can operate readily with reactive gases, such as oxygen, without rapid electrode erosion problems. The specific related art is cited at the end of the specification.
The ability of microwave sources/torches to process a wide variety of materials is similar to laser based processes such as cutting and welding. The trade-off between plasma torches and lasers is that laser cutting, for example, gives a cut width of 0.01-0.05 mm, and known microwave torches are limited to cut widths of several 100's of microns. However, an important advantage of the microwave plasma torch is that its capital investment can be considerably less than the equivalent laser processing technology. Plasma torches can also be combined with laser technology to produce a hybrid cutting tool. In these hybrid tools the plasma jet has a laser beam propagating down the center of the jet. Because the plasma jet is heating the material, almost all the laser energy can be applied to the energy needed for high-precision deep cuts. This hybrid torch/laser technology thus is able to use a less costly laser system while still achieving the processing rate of a more costly laser system. There is a need for miniature microwave torches.
U.S. Pat. No. 4,611,108 to Leprince et al. discloses a microwave plasma torch excited by means of microwave energy delivered by a rectangular cross-section waveguide which is transversed by a delivery tube extending to the discharge outlet. A rectangular piston in the waveguide is capable of displacement in a sliding motion to form a short-circuit and is one of the factors to permit impedance-matching of the system. The delivery tube is hollow to carry a gas to the end which narrows to a discharge outlet having a 2 mm internal diameter. There is no liquid cooling system.
U.S. Pat. No. 6,184,982 to Karanassios discloses an in-torch vaporization sample introduction system for sample introduction into a spectrometer. The inductively-coupled plasma device (Fassel-type torch) includes a plasma with a central channel and load coil, fed by Argon though outer and intermediate feed channels in an enlarged gas tube.
U.S. Pat. No. 6,213,049 to Yang discloses a nozzle-injector for plasma deposition of thin-film coatings. The nozzle-injector utilizes an arc torch as the plasma generator and releases the plasma into a vacuum chamber reactor towards a substrate to be coated.
U.S. Pat. No. 6,218,640 to Selitser discloses a method of processing a semiconductor device using an inductive plasma torch. An inductive coil is used to apply an oscillating magnetic field to a plasma confinement tube to ignite and sustain a plasma.
U.S. Pat. No. 6,397,776 to Yang et al. discloses an expanding thermal plasma system for large area chemical vapor deposition. Each of a plurality of plasma generating means produce an expanding plume of plasma which impinge on a substrate within a deposition chamber for the purpose of producing a coating on the substrate.
While the related art teach microwave plasma torches, there still exists a need for a miniature microwave torch which provides a small plasma discharge.
OBJECTS
An object of the present invention is to provide a miniature microwave plasma torch apparatus that operates near or at atmospheric pressure for use in materials processing. The miniature plasma torch can also be applied to plasma torch spectroscopy applications. The apparatus provides a wide range of flow rates so that discharge properties vary from diffusional flow of radicals for gentle surface processing to high velocity, approaching supersonic, torch discharges for cutting and welding applications. It is particularly an object of the present invention to provide a miniature microwave plasma torch with a materials processing spot size of about 0.25 mm to a few mm's. Another object is to provide a hybrid microwave plasma torch/laser apparatus for materials processing.